1. Field of the Invention
The present invention relates generally to interface circuits for focal plane arrays (FPAs) and, specifically, to a self-adjusting adaptive amplifier circuit that uniquely provides high charge-handling capacity for optimally coupling IR detectors to multiplexing readouts in high-density staring FPAs.
2. Description of the Related Art
Optical sensors transform incident radiant signals in any spectral wavelength region, but most specifically in the near infrared (NIR; λ=0.8-2 μm), short wavelength IR (SWIR; λ=2.0-2.5 μm), medium wavelength IR (MWIR; λ=2.5-5 μm), and long wavelength IR (LWIR; λ=5-30 μm) bands into electrical signals that are used for data collection, processing, storage and display, such as real-time video. For high-quality imaging of various scenes without concern for ambient light, the MWIR and LWIR bands are often used interchangeably. However, MWIR infrared detector systems typically require sophisticated signal processing algorithms to accommodate the large dynamic changes in background information that result from the relatively high contrast and large solar influence of the scene radiation. Detectors operating in the preferred long wavelength infrared (LWIR) spectral band, on the other hand, can attain the same or greater thermal sensitivity with reduced signal processing complexity. This is especially true in the 8 to 12 μm wavelength atmospheric window, which is optimum for imaging many terrestrial scenes. As a result, infrared detection and tracking can be accomplished using smaller, more cost-effective sensors having LWIR focal plane arrays.
Unfortunately, the limited ability of the multiplexing readout circuits creates practical design constraints on LWIR focal plane arrays that in turn severely limit system performance. The result is degradation in signal-to-noise ratio by >10× below the theoretical limit. In the readout portion of a focal plane array, each pixel has a preamplifier to couple the signal from each detector into the respective unit cell. The corresponding readout site must perform several functions that are difficult to simultaneously incorporate in the small amount of “real estate” typically available on such a signal-processing chip. Ideally, each detector/amplifier cell of an FPA should include the following: 1) a detector interface stage that provides low impedance at a uniform operating bias; 2) an integration capacitor with large charge-handling capability; 3) a stage for uniform suppression of the background if integration capacity is inadequate; 4) facility for low-power multiplexing and reset; 5) an output buffer capable of driving the bus line capacitance for subsequent multiplexing of the electrical signal at video rates; and 6) a sufficiently large transimpedance to enable sensor-limited rather than camera-noise-limited performance; i.e., the output-referred noise level of the shot noise at the lowest background must be easily measurable by conventional camera electronics. Note that any focal plane array (not just an LWIR array, but for instance a high operating temperature MWIR array) whose operation generates large amounts of charge to be integrated in the input cell will suffer from these multiplexer limitations even beyond the natural limits of the detector inputs.
Staring LWIR FPAs in formats up to 1024 by 1024 have now been demonstrated in the prior art. However, these LWIR devices are typically coupled to conventional MWIR readout circuits, which have several deficiencies that compromise system performance. The limited charge-handling capacity, for example, supports camera sensitivity no better than that achieved by a typical MWIR FPA. This obviates a key benefit of operation in the LWIR spectral band. Moreover, prior art devices limit capability for reducing pixel pitch and increasing pixel density. If the pixel pitch and detector/amplifier cell real estate are reduced in prior art devices, the performance limitations are further exacerbated.
Given the current photolithographic state-of-the-art and the limited chip area, there is insufficient detector/amplifier cell real estate to integrate even the most important features including the ability to directly handle all the charge that can be generated during the full frame time. Nevertheless, because small cells are necessary for FPAs with high pixel counts that can be used with compact optics, the readout circuit must be integrated in as little chip real estate as possible. Thus, there is a need for a compact amplifier having characteristics that are better optimized for use in staring LWIR FPAs.
U.S. Pat. No. 5,128,534 teaches the technique of biasing a capacitor with a variable voltage source to increase the amount of charge that can be effectively integrated on an integrating capacitor (of the typical size that can readily fit into a standard unit cell). This improves the signal-to-noise ratio of the focal plane array by clipping excess signal. The enhancement in capacity is a trade-off for a nonlinear dynamic range; this non-linearity is not optimum for LWIR imaging since the signal of interest is a small fraction of the background radiation.
U.S. Pat. No. 5,055,667 and U.S. Pat. No. RE34,802 also disclose nonlinear techniques for visible CCD imagers. These methods control the dynamic range of a CCD in a manner somewhat analogous to U.S. Pat. No. 5,128,534 by providing a sink region to dispose of excess charge from the photogate region by clipping any signal above the potential set by a control gate where the potential can be similarly modulated during the exposure period. Such predetermined fluctuation can facilitate various transfer functions including logarithmic behavior. Such characteristics are not optimum for LWIR imaging.
U.S. Pat. No. 5,382,977 proposes an alternative that effectively enhances the linear charge-handling capacity via electronic scanning. Since the total charges accumulated during a typical frame time ({fraction (1/60)}th second) can exceed 1010 carriers while the typical integration capacitor can only handle on the order of mid-107 carriers, this method accepts the ˜100× disparity between the two to extend the linear capacity to approximately 109 carriers. Unfortunately, all the available charges for each frame time are not used so maximum sensitivity is impossible. The inefficiency translates to a proportional reduction in the effective quantum efficiency of the imaging sensor.
U.S. Pat. No. 5,146,302 again teaches skimming in a CCD to enhance the linear charge-handling capacity. To improve efficacy, a sampling circuit comprising a tandem input is used to generate a prescribed skimming voltage. However, the non-uniformity in threshold voltages between the input gates of the main input and the sampling input sets a limit on the amount of charge that can be skimmed. This limit is typically no more than 3 times the instantaneous charge-handling capacity. The linear capacity is thus effectively increased to the order of 108 carriers. A further improvement of two orders of magnitude is still needed.
Finally, U.S. Pat. No. 6,064,431 discloses a circuit for skimming to linearly enhance the effective charge-handling capacity. This circuit, shown in FIG. 1, provides automatic control of the skimming charges to be transferred. Here the capability for predetermined sinking of excess signal, as taught by U.S. patent '534, '667 and '802 noted above, is combined with conventional skimming, as taught by the '302 patent, to further enhance the effective charge-handling capacity. In principle, the 2× to 3× enhancement of '302 patent can be enhanced to yield several orders of magnitude enhancement in charge-handling capacity. Unfortunately, the improvement is again non-linear and thus not optimum for infrared applications.